This invention relates to a cryosorption pump having an improved construction for obtaining a high vacuum condition in a pumping system.
A usual cryosorption pump is provided with a pumping chamber to be evacuated to obtain a high vacuum condition and with a cryosorption member having an extremely low temperature surface, i.e. cryosurface, disposed at substantially the central portion of the pumping chamber so as to be cooled to a temperature of about 4.2 K. (absolute temperature) by means of liquid helium. However, helium is not condensed and evacuated on the cryosurface at the temperature of about 4.2 K. for the reason that helium has a high equilibrium vapour pressure. In order to evacuate the helium, the cryosurface of the cryosorption member is coated with an adsorbent such as active carbon and molecular sieves, or a condensed gas layer composed of a gas such as Ar or CO.sub.2 gas.
In the cryosorption pump of the type described above, a further cryocondensation member (or members) on which no adsorbent is applied is (or are) arranged so as to surround the cryosorption member on which the adsorbent is applied. The former mentioned cryocondensation member (or members) serves (or serve) to preliminarily condense gases other than helium which are condensed at a temperature of about 4.2 K. thereby to reduce an excessive gas load on the cryosurface of the later mentioned cryosorption member on which the adsorbent is applied.
For example, in a nuclear fusion system in which the use of the cryosorption pump will be required, a large amount of gases such as deuterium, tritium and helium are to be evacuated. When the conventional cryosorption pump is used for the nuclear fusion system, deuterium and tritium gases will be condensed on the cryosurface on which no adsorbent is applied, whereas helium will be adsorbed on the cryosurface on which the adsorbent is applied. However, as is known, for example, tritium emits .beta.-rays (a kind of energetic electron) each having maximum energy of 18.6 KeV and average energy of 5.96 KeV when the tritium decays with a half life of 12.3 years. Since the .beta.-rays are emitted from the tritium once condensed on the cryosurface substantially directly into the atmosphere in a case where no magnetic field is generated therein, some of .beta.-rays impinge on the cryosurface on which the adsorbent is applied. The helium gas adsorbed on the adsorbent is usually very weakly adsorbed, so that a large amount of the helium is desorbed by the impact of the .beta.-rays, which is generally called "electron impact desorption". This phenomenon adversely results in the lowering of the effective pumping speed of the helium on the cryosurface. This is not advantageous for the cryopumping operation.
Moreover, in a cryopumping chamber of a nuclear fusion system, secondary electrons are emitted by the impact of the .beta.-rays or fast particles such as neutrons against the wall surface of the chamber and electrons are also generated therein by the Compton effect. These electrons then impinge on the cryosurface on which the helium is adsorbed, thus separating the helium. This is a significant problem for the cryopumping system.